This invention relates to communication line monitoring systems and, more particularly, systems designed for monitoring communication line integrity between the central control station and plural remote stations in a security network. Such security networks may be used, for example, for limiting access to various industrial or commercial facilities or for monitoring the movement of personnel throughout a facility.
In the prior art, such systems have provided a limited degree of line security, but only through substantial waste of electrical energy. Specifically, systems in the past utilizing plural remote terminals have generally operated on a polling basis. That is, a central computer station polls the remote stations, generally by station address, in a particular order to determine whether each of the stations has data requiring communication to the central computer. Such systems thus transmit polling signals from the central computer over a polling signal line, the signals being sent sequentially to poll all the remote terminals in the system in sequence. If no response is received from a given terminal indicating a need to communicate, the next terminal in order is polled by the central computer.
Obviously, if all of the communication lines to one of the remote terminals has been cut, the polling signal transmitted by the central computer for that terminal will produce no response. Since, however, a lack of transmitted data is the expected response if the terminal does not have data for communication to the central processor, in order to provide some degree of line security in this type of communication system, the prior art has utilized a transmission code in which the non-transmission mode provides a continuous flow of current in the data loop between the central processor and the remote terminal. Thus, data is communicated in this network by open circuiting this continuous current flow for short periods of time in a binary coded fashion. By utilizing this continuous current flow during non-transmission periods, it is possible to detect an open circuit data line. Since the central computer interprets the lack of current produced by open circuited data lines as a data condition, it stops its polling operation, and waits for the remainder of the data transmission which, of course, in the case of open data lines, never occurs. Under these circumstances, prior art systems have provided an indication of open circuit data lines which were monitored by noting a single bit data condition where no further data is received.
Such systems, of course, could not provide security for the lines which would detect a shorted data line condition, since a short data line would be interpreted as a normal condition in which current was flowing during non-transmission periods. Furthermore, in systems in which separate polling and data lines are utilized, such prior art security was unable to detect open circuit or short circuit configurations in the polling lines, since the data line in that situation would provide a current return path which is indistinguishable from the normal response when a polling signal is received but no data needs transmission.
It should be understood that a typical environment for such security systems is one in which a central computer located in an industrial or commercial facility is used to give selective access at various doors throughout a facility. Each of these doors will be provided with a remote terminal into which various personnel can place a pass card. On reading the pass card, the remote terminal transmits the identification of the person wishing to gain access to the central computer in response to polling signals from the computer which are received periodically. Thus, this remote terminal must wait its turn to communicate its data with the main computer. The main computer, in response to the identification of the individual wishing to gain access, will provide data signals which direct the remote terminal to grant or deny access by opening or not opening the particular lock on the door next to the remote terminal. Security is important in such systems to prohibit clandestine access to certain portions of a facility, either by employees who, although they have cards, are not entitled to access to this particular facility portion, or to persons who are not entitled to any facility access whatsoever.
It will be understood that the prior art systems using a continuous current flow as the non-transmission mode provide only minimal line security, but at the same time utilize very substantial amounts of electrical power to maintain the constant current flow in all of the remote lines.